Civil Engineering Reference
In-Depth Information
The back of the
wall should not be
rendered
Hoods, bellcasts and sills should
overhang the remainder of the
rendering by at least 50 mm with the
throating clear of the surface. Sills
should project beyond reveals (Figure
9.11 on page 249). Rendered sills are
not recommended, especially those
under large windows, as these tend to
be continuously damp saturated by
run-off from the window above.
Main performance
requirements and defects
Choice of materials for structure
The types of background, their
strength and condition, are crucial to
successful rendering as much as to
appropriate specification, including
render mix (see the feature panel
below), numbers of coats, and the use
of additives and special surface
treatments.
If the background is deficient in
some respect, it may be possible to
introduce a surface treatment, or
overlay with an armature of some
kind which allows a wider choice of
solutions. For a background of
varying materials, a mix suitable for
the weakest material should be
specified or the background covered
with expanded metal lathing.
Masonry backgrounds to be
rendered directly should be stronger
than the rendering. Each subsequent
coat should be not stronger than that
preceding. Weak or eroded
backgrounds may still be rendered,
though they will need surface
preparation (see the feature panel
opposite).
Throating
clear of render
Roof finish
Parapets and screen walls
Parapet walls to be rendered should
be of cavity construction. Rendering
should not be applied to the backs of
walls which are rendered on the front
(Figure 9.12).
Rendering stop in
stainless steel
String course
Figure 9.12
Rendering parapets
Base of walls
Walls below DPC level will be wetter
than those above, so specifications
for rendering below the DPC must be
chosen with care (Figure 9.13).
Renderings below DPCs should be
physically separated from those
above to avoid salts carried by rising
damp crystallising behind the
rendering (Figure 9.14 on page 252).
External corners
External corners can be reinforced
with expanded metal beading in
stainless steel, or even plastics; if,
though, differential movement has
not been allowed for, it may just
transfer the inevitable crack from the
arris to the edge of the lathing.
Cements and limes
●
Cements should comply with
BS 12
(234)
, BS 146
(235)
or
BS 4027
(236)
●
Limes should comply with
BS 890
(237)
Figure 9.13
Rendering below the DPC has failed and
fallen away, perhaps understandably in view
of the condition of the substrate!
Mixes suitable for rendering
Masonry
cement:sand
Cement:sand with
air-entraining agent
Cement:lime:sand
Cement:ready-mixed lime:sand
Ready-mixed
lime:sand
Cement:ready-
mixed material
1
3
1
12
1
3
Designation I
1
4
1
4-4
1
9
1
4-4
1
1 3-4
1 2 -3
1
1
1
1
Designation II
2
2
2
2
2
1
1 6
1
6
1
5-6
1 4-5
Designation III
1 5-6
1
2 9
1
4
1
1
1
1
8-9
1 7-8
1 5 -6
Designation IV
2
2
2
1
3
10-12
1
4
1
10-12
Designation V
These mixes can also be used for top coats but may be less tolerant of movement.
